The paraoxonase (PON) gene family consists of three family members, PON1, PON2 and PONS. PON genes are implicated in the pathogenesis of several inflammatory diseases including atherosclerosis. All three PON proteins possess antioxidant properties and lactonase activities;however, the physiological substrates for PON proteins and their mechanism of action remain unknown. Our long-term goal is to understand the role of PON2 and PONS proteins in physiology and disease. During the last grant period, we have developed transgenic/knockout mouse models for PON2 and PONS genes, and provided the first in vivo evidence for the anti-atherogenic functions of PON2 and PONS proteins. We have also demonstrated that oxidized phospholipids or lipid peroxidation products are not direct substrates for purified PON enzymes, suggesting that direct inactivation of atherogenic oxidized phospholipids is not a key mechanism of action of PON proteins. We propose that the anti-atherogenic activities of PON proteins are a result of their activity towards a class of unidentified pro-inflammatory lipo-lactones that participate in LDL oxidation. Interestingly, in a very exciting turn of events, we recently discovered that PON2 and PONS proteins degrade gram-negative bacterial density-dependent sensing molecules termed acyl homoserine lactones (AHL) involved in gram-negative virulence. AHLs are lipo-lactones. Since gram-negative virulence is well established in the pathology of inflammatory diseases, including atherosclerosis, we propose that protection against pro-inflammatory pathways mediated by gram-negative virulence factors, such as AHLs, and other unidentified pro-inflammatory lipo-lactones, is a key physiological function of PON proteins. We will test this hypothesis in PON2 and PONS transgenic/knockout mouse models under two specific aims to 1) investigate the molecular mechanisms and function of PON2 and PONS in mitigating the development of atherosclerosis and 2) determine whether PON2 and PONS play a role in host defense against pro-inflammatory quorum sensing molecules and gram-negative bacterial infection. We hypothesize that PON2 and PONS are a novel class of lactonases that degrade i) atherogenic lipo-lactones associated with atherosclerosis, and ii) pro-inflammatory acyl homoserine lactones associated with gram-negative bacteria, thus preventing the pathogenesis of inflammatory diseases.